The present invention relates to a laser system comprising a gain medium for providing and amplifying a laser beam within an optical resonator.
Molecules, and in particular gas molecules, are mainly investigated by spectroscopy. Two different spectroscopic methods, i.e. infrared absorption and Raman scattering, are generally applied for airway gas monitoring. The most common and widely spread measuring type is the infrared absorption, since it provides a robust and simple system with reliable accuracy. Disadvantageous, however, is that the infrared absorption is not flexible for upgrading to other molecules. Raman scattering overcomes that disadvantage because each molecule provides its own characteristic scattering signal. In addition and in contrast to the infrared absorption, the wavelength of the excitation light can be chosen flexibly. The drawback of the Raman scattering, however, lies in its minor effect, meaning that an excitation power of a light beam will create only a very low Raman signal (e.g. an excitation power of 1 W will create a Raman signal of 1 pW).
For medical purposes, such as respiratory or anesthetic gas monitoring, Raman scattering has been investigated as shown e.g. by Van Wagenen et al in "Gas Analysis by Raman scattering", Journal of Clinical Monitoring, vol. 2 No. 4, October 1986.
Because of the minor effect in Raman scattering, the optical output power of the excitation light should be selected as high as possible. In addition, to achieve a good resolution of the molecule spectra, the excitation light source should be a narrow band source with a good wavelength and power stability. Thus, laser sources are commonly used as excitation light sources, whereby for reasons of compactness, lifetime and price, semiconductor lasers are normally superior to solid state or gas lasers. However, semiconductor lasers exhibit, in contrast to solid state and gas lasers, the disadvantage of a low internal circulating optical power and a low coupled out optical power.
A known solution for increasing excitation power for Raman scattering is disclosed in U.S. Pat. No. 5,153,671 and U.S. Pat. No. 5,245,405 for a gas analyzing system. A gas analysis cell employing Raman scattering is positioned within a single optically resonant cavity. The gas flow is directed into the cavity and analyzed within the gas analysis cell. FIG. 1A shows in principle such a laser system 10 in the gas analyzing system of U.S. Pat. No. 5,153,671. The laser system 10 comprises a laser cavity 20 between a first mirror 30 and a second mirror 40. A gain medium 50 provides and amplifies a laser beam 60 which serves as an excitation beam in a gas analysis cell 70 within the laser cavity 20. The first mirror 30 may also be part of the gain medium 50.
In a more sophisticated solution for increasing excitation power, in particular when semiconductor lasers are used as excitation sources for Raman scattering, the optical output power of the excitation laser is coupled into an external resonator as shown e.g. in U.S. Pat. No. 5,642,375 or U.S. Pat. No. 5,684,623 by the same applicant. FIG. 1B shows in principle such a coupled laser system 80. The coupled laser system 80 comprises the laser cavity 20 between the first mirror 30 and the second mirror 40 and the gain medium 50 providing and amplifying the laser beam 60. An external cavity 90 is provided between the second mirror 40 and a third mirror 95, and is optically coupled to the laser cavity 20. The laser beam 60 serves as excitation beam in the gas analysis cell 70 within the external cavity 90. By applying low loss mirrors with different reflection coefficients for the mirrors 30, 40 and 95, as described e.g. in U.S. Pat. No. 5,642,375, a very high built-up optical power inside the resonator of the external cavity 90 can be achieved. For example, a 10 mW semiconductor laser beam 60 is capable of pumping the external cavity 90 up to several hundreds of Watts. U.S. Pat. No. 5,432,610 further discloses a passive, purely optical locking of a laser diode on an external resonator.
Using such an external pumped resonator, as depicted as the laser system 80 in FIG. 1B, for probing an unknown gas sample in the external cavity 90 will in particular provide enough optical power to excite a Raman signal well above the sensitivity limit of optical sensors. Optical sensors can simply be photodiodes, charged coupled devices or other image sensors for more sophisticated applications.
As well in the single cavity laser system 10 as in the coupled cavity laser system 80, the gas analysis cell 70 represents the principal possibility of probing a gas sample, whereby the gas sample can be analyzed in a specific (separated) environment or directly in the respective cavity. Probing the gas sample can either be accomplished `offline`, i.e. the gas sample is taken and analyzed later (e.g. in a defined environment), or `online`, i.e. the gas sample is directly provided to the gas analysis cell 70 and analyzed. The latter case, in particular, allows monitoring of a gas flow such as a respiratory or anesthetic gas. Online gas monitoring, however, requires an increased effort with respect to stabilizing the laser system.
If there are no changes of the applied active and passive components of the laser system, e.g. laser system 10 or 80, and as long as the environmental conditions remain unchanged, the light beam 60 (in the laser cavity 20 of FIG. 1A or in the external cavity 90 in FIG. 1B) will substantially remain at constant power. The light beam 60 circulating in the laser cavity 20 and the external cavity 90 comprises one or more (longitudinal) optical modes determined by the components of the laser system and the specific environmental circumstances within the respective cavity/cavities. Associated with each optical mode are a defined wavelength and a defined roundtrip phase shift. The gain medium 50 supports the optical mode(s) that match(es) the required wavelengths and provides the necessary phase, thus leading to a high intensity build-up light beam 60 at the supported optical mode(s).
It is to be understood that semiconductor type lasers generally only support one optical mode at a time, while other laser types (e.g. gas laser) may support more than one optical mode concurrently. For the sake of simplicity, only semiconductor type lasers, supporting only one optical mode at a time, shall be considered in the following. However, it is clear that the principals as illustrated herein are applicable for multi-mode concurrently supporting lasers accordingly.
In the coupled cavity system of FIG. 1B, a locking mechanism between the two resonators has to take place. To achieve substantial amplification and stability in the external cavity, the feedback of the external cavity into the laser cavity has to be adjusted, so that the laser diode radiation emits coherent radiation with a bandwidth and a wavelength to actively support the external cavity 90 at a cavity resonant frequency. This process is called hereinafter "optical locking".
When a change of the applied active and passive components (e.g. of the optical path length) of the laser system occurs and/or the environmental conditions change, the currently supported optical mode does not match anymore the required wavelength and phase shift, and the laser system has to `find` another optical mode matching the changed resonating conditions within the laser system. Thus, the light beam 60 can suddenly extinguish (albeit temporarily) until a new optical mode is built up fitting to the changed cavity properties. This leads to a (significant and in most cases unwanted) variation of the optical power of the optical beam 60 over the time.
FIG. 2 shows an example of a variation of the optical power over the time in the external cavity 90 in an arrangement according to FIG. 1B. The power variations in the measurement example of FIG. 2 were possibly induced by temperature or other environmental variations causing modifications in the optical lengths of the laser system 80. When applied for Raman scattering, the power variation leads to different results for the Raman scattering measurement due to a variation in the exciting power.
A known possibility for avoiding environmental disturbances is disclosed in U.S. Pat. No. 5,245,405 for the gas analyzing cell as discussed above. A pressure control system eliminates pressure variations in the gas cell regardless of changes in restriction, gas viscosity and barometric pressure. Maintaining a constant pressure in the gas cell makes the system more stabile, since optical alignment through the gas cell is sensitive to gas pressure. Other known solutions suggest to arrange mechanical pumps for providing the gas flow as far as possible away from the gas analyzing cell in order to reduce disturbances by the pump as much as possible. However, although all of those solutions refer to gas analyzing purposes, they have been proved unpractical in several cases since the gas flow, in particular for respiration gases, is hardly controllable and always provides a source of disturbances, e.g. due to variations in the gas compounds.
A solution for an actively locked external optical resonator 90 is an external servo control loop, which changes the characteristics of the laser cavity 20 in a way that this cavity 20 remains locked to a single external cavity mode with a constant optical power. Parameters to change the characteristics of the laser cavity 20 are current and temperature of the active laser medium 50 as well as mechanical changes of the laser cavity 20. This solution, however, requires highly sophisticated countermeasures to ensure stabile conditions, and because of the necessary resolution, such a servo control loop can become very expensive and costly.